Magnetic Resonance Imaging (MRI) measures tissue-specific responses to a radio frequency (RF) stimulus in a strong main magnetic field (B0). Specifically, the magnetization of tissue is aligned with B0. An initial RF pulse tips the magnetization out of this alignment and rotates with a tissue-specific RF frequency resulting in a signal that is picked up with a receiver coil. Additional magnetic field gradient pulses (G) are used to spatially encode the RF signal that, in turn, is used to reconstruct an image.
MRI relies on a very homogeneous magnetic field B0. Unfortunately, the magnetic field of a clinical MR scanner typically exhibits inhomogeneity as a result of imperfections of the coil generating B0 and susceptibility interfaces between different types of tissue (e.g., lung and liver) that are highly patient dependent. The inhomogeneity of B0 can be compensated by adding additional magnetic fields generated by dedicated coils (shim coils) that are often described by a constant, linear, 2nd order and even higher order terms. Modern clinical scanners typically employ at least 2nd order shims.
Before acquiring clinical images in a patient, the existing inhomogeneity is often measured and corrected. MRI scanners estimate the magnetic field B0 by applying a dedicated MR sequence. Historically, a three dimensional (3D) multi-echo steady state sequence (e.g., Double Echo Steady State or “DESS”) has been used. The phase evolution between the echoes is proportional to B0 and the resulting phase map can be converted into a field map. Finally, the field map is being used to derive shim currents that generate additional magnetic fields that compensate the inhomogeneity of B0.
More recently, multi-slice two dimensional (2D) multi-echo sequences have been introduced to generate a field map by acquiring a stack of 2D slices covering the area of interest. The data acquisition of a single 2D slice is faster than typical bulk, respiratory, cardiac and peristaltic motion of patients and results in a more reliable phase map than using a 3D DESS sequence and ultimately results in improved image quality. More advanced approaches implement different sets of shim currents for different portions of the scan volume (e.g., for an interleaved 2D multi-slice data acquisition) that further reduces effects from motion and ensures continuity of B0 across different portions of the scan volume. The shimming sequence is performed at least once per patient exam but typically repeated throughout the patient exam if certain criteria are being met such as changes of the region of interest, receiver coil configuration, etc.